AP_Arming: Use SI units conventions in parameter units

Follow the rules from:
http://physics.nist.gov/cuu/Units/units.html
http://physics.nist.gov/cuu/Units/outside.html
and
http://physics.nist.gov/cuu/Units/checklist.html
one further constrain is that only printable (7bit) ASCII characters are allowed

libraries/AP_Arming/AP_Arming.cpp

@@ -57,7 +57,7 @@ const AP_Param::GroupInfo AP_Arming::var_info[] = {
     // @Param: MIN_VOLT
     // @DisplayName: Minimum arming voltage on the first battery
     // @Description: The minimum voltage on the first battery to arm, 0 disables the check.  This parameter is relevant for ArduPlane only.
-    // @Units: Volts
+    // @Units: V
     // @Increment: 0.1 
     // @User: Standard
     AP_GROUPINFO("MIN_VOLT",      4,     AP_Arming,  _min_voltage[0],  0),
@@ -65,7 +65,7 @@ const AP_Param::GroupInfo AP_Arming::var_info[] = {
     // @Param: MIN_VOLT2
     // @DisplayName: Minimum arming voltage on the second battery
     // @Description: The minimum voltage on the first battery to arm, 0 disables the check. This parameter is relevant for ArduPlane only.
-    // @Units: Volts
+    // @Units: V
     // @Increment: 0.1 
     // @User: Standard
     AP_GROUPINFO("MIN_VOLT2",     5,     AP_Arming,  _min_voltage[1],  0),